In connection with engine control and powertrain systems and body systems for controlling air conditioners and the like, in transportation vehicles, the contents of control have become more complicated and more systems have been becoming necessary for control. Attendant on this trend, the number of electronic control units (ECU) mounted on the vehicle is increasing every year, and the number of electronic elements mounted therein is also increasing. As a plurality of electronic elements and parts of different heights generate heat, heat-conductive materials are now indispensable to efficient transfer of the heat to die-cast aluminum casings.
Further, since it is recently required to mount more electronic elements and parts within a limited space, their mounting environment (e.g., ambient temperature, humidity, angle, thickness, etc.) is more diversified. In the case of engine ECU, for example, electronic elements and parts have come to be more often installed vertically in the engine room. Thus, there are more possibilities that heat-conductive materials are positioned vertically in a site where both vibration and high temperature are applied.
In connection with the use of heat-conductive materials in such environments, there have been several proposals made to prevent the heat-conductive material between the heating body and the cooling body from sagging and falling down, by the use of, for example, a heat-conductive silicone adhesive material, a heat-conductive potting material, or a room temperature-curable heat-conductive silicone rubber composition as the heat-conductive material (see JP-A 1996-208993, JP-A 1986-157569, JP-A 2004-352947, Japanese Patent No. 3543663, and Japanese Patent No. 4255287: Patent Documents 1 to 5).
However, since all these materials may have a high hardness and would adhere to substrates, they have drawbacks in that they are poor in re-workability and apply stresses to the heat-generating electronic elements. In addition, since the heat-conductive material cannot withstand the repeated stress due to thermal strain, it may peel from the heat-generating element or crack, leading to a rapid increase in thermal resistance.
In view of this, an addition one part heat-conductive material was discovered (JP-A 2003-301189: Patent Document 6) which has previously undergone a thermal crosslinking reaction to get a high viscosity (to maintain softness or flexibility) during preparation of the heat-conductive material so that the material is not liable to sag. Since this material is quite soft or flexible despite the high viscosity, its influence of exerting stress on the electronic elements is weak as compared with high-hardness materials. Since this material freely deforms and conforms in shape to a rugged surface (projected and recessed surface), it can be suitably applied to electronic elements of different heights. However, there is naturally a tradeoff problem that the coating with the material is difficult because of its high viscosity.
In recent years, a heat-conductive silicone composition lowered in viscosity as compared with the addition one part heat-conductive material has been developed (JP-A 2009-286855: Patent Document 7). Its viscosity is still high, and, accordingly, there has been a request for a heat-conductive silicone composition that has better workability and sag control.
The above-mentioned problem can be solved by an addition one part heat-conductive silicone composition (JP-A 2002-327116: Patent Document 8). This composition is readily dischargeable prior to heat curing, has a certain degree of re-workability even after heat curing, does not sag after curing, remains as a comparatively soft or flexible rubber even after curing, and thus can also play the role of a stress relaxing agent. Nevertheless, this addition one part heat-conductive silicone composition still has a problem to be solved. The problem is that when the addition one part heat-conductive silicone composition is further lowered in viscosity, the composition becomes flowable so that it may spread over the electronic element immediately after discharging, failing to establish a heat-dissipating passage if a substantial space is defined between the electronic element and the cooling substrate.
Under such circumstances, an addition one part heat-curable type heat-conductive silicone grease composition has been proposed which has high shape retention properties despite a low viscosity initially and is soft or flexible after curing. Since cold storage or refrigerating storage vessel is normally needed for storage of addition one part heat-conductive silicone compositions, however, there has been the problem of storage cost. In view of this, there has been a request for an addition one part heat-conductive silicone composition that can be stored at normal temperature, desirably at a temperature of up to 40° C. (JP-A 2013-227374: Patent Document 9).
As the above-mentioned other curing means, an organopolysiloxane gel composition has been proposed (Japanese Patent No. 3865638: Patent Document 10) which is photoactive by ultraviolet (UV) rays and in which a platinum complex curing catalyst is used. While the document describes the addition of an inorganic filler as an optional component, however, there is no description as to the amount of the inorganic filler added, and there is no description about thermal conductivity. Besides, this composition has a difficulty as to storage stability on a one part basis.